Because parasitosis hinders stable production in mariculture, it is an extremely serious problem. Cryptocaryon irritans, which belong to the subkingdom Protozoa Phylum Ciliophora, parasitize sea fish and cause white spot disease. As a result, parasitized fish weaken to the point of death. In recent years, the parasitism of Cryptocaryon irritans on cultured fish has become a serious problem in the fish culture industry of Japan. Cryptocaryon irritans has thus far been reported as being parasitic to Paralichthys olivaceus, Seriola dumerili, Seriola quinqueradiata, Pagrus major, and Takifugu rubripes. Cryptocaryon irritans and Ichthyophthirius multifiliis are two types of ciliate protozoan that manifest as white spots on fish. The bionomics of Cryptocaryon irritans is similar to the bionomics of Ichthyophthirius multifiliis, which attracts white spot disease, wherein it repeats the stages of a trophont (imago) living in the body of a fish to which it is parasitic until the host dies, entirely leaving the fish to emerge as a protomont in the seawater, the protomont subsides and attaches to the sea bottom, where it ceases activity in a tomont (cyst, also known as a resistant egg), it is discharged again from the tomont, and it becomes a theront (larva) floating in the water in search of a fish that would become a host to which it can be parasitic. The trophont becomes parasitic on the epidermis and the branchial epithelial tissue of the fish and grows to a visible size, taking in nourishment from the body of the host.
The longevity of a theront is short and it can fairly readily be killed with a comparably weak drug. The phoront (parasitic larva) gains entrance to the epidermal layer near the dermis of the body surface of a fish and grow to an imago (trophont), so it is difficult to exterminate with drugs from the exterior of the body surface of these fish. A cyst is also covered by an outer shell, so it is difficult to exterminate it with drugs. Therefore, to exterminate the ciliate protozoa, it is necessary to periodically repeat a medicated bath in order to reduce the number of ciliate protozoa by exterminating the theront and trophont that have not infiltrated the dermal layer of the body surface and ultimately cut the multiplication cycle. Methods of extermination that take time in this way have been adopted, but there are cases in which it does not control damage to the extent of preventing the death of the fish.
As for orally administered medicine, lysozyme chloride has been authorized as fisheries medicine for combating White spot disease in Pagrus major. In addition, lactoferrin has also been marketed as an additive for fisheries (Patent Document 1). One report states that, when 40 mg of lactoferrin per 1 kg of fish body weight was orally administered per day during a 28-day experimental period, no infection was observed, as opposed to the majority of the fish dying after being given feed without any such additive. The mode of action of lysozyme chloride and lactoferrin is thought to be the increase in the non-specific biophylactic ability of the host fish, but the results are limited (Non-Patent Document 1). An antiparasitic agent that can directly exterminate parasites via oral administration is highly anticipated.
Sulfonamide is known as an antibacterial agent that has a sulfanilamide group, and the mechanism thereof is known to hinder the folate synthesis of bacteria. In addition, trimethoprim, ormetoprim, etc., are known as antagonists of folate metabolism, being drugs that demonstrate antibacterial action in the mechanism related to folic acid similar to that of sulfonamide. Methotrexate and aminopterin are derivatives of folic acid, which are variously used as anticancer agents and mouse poison.
In the category of antimicrobial/antibiotics, the combination preparations of sulfadimethoxine, sulfamonomethoxine, sulfamonomethoxine, and ormetoprim have been authorized as pharmaceutical products for fisheries. Sulfonamide was originally an antibacterial agent, but it is now known to be effective against malaria in humans, and against animal and avian coccidiosis, etc., as a pharmaceutical product for animals (Patent Document 2).    Patent Document 1: Japanese Patent Application Publication No. H9-301807    Patent Document 2: Japanese Patent Publication No. S51-16346    Non-Patent Document 1: Kazuo Ogawa (2004): Protozoal diseases. Infectious and parasitic diseases of fish and shellfish (eds. H. Wakabayashi, K. Muroga). Koseisha-Koseikaku. pp. 285-320.    Non-Patent Document 2: Dickerson, H. W., Dawe, D. L. (1995) Ichthyophthirius multifiliis and Cryptocaryon irritans (Phylum Ciliophora). In: Woo, P. T. K. (Ed), Fish Diseases and Disorders. Protozoan and Metazoan Infection, vol. 1, CAB International, UK, pp. 181-227.